1. Field of the Invention
The present invention relates to a viewing optical instrument (such as a telescope, a monocular, binocular or field glasses) in which the eyepiece can be brought into focus entirely on a plane which is not normal to the optical axis of the objective optical system.
2. Description of the Related Art
In conventional viewing optical instruments such as telescopes, monoculars, binoculars or field glasses, the eyepiece can be brought into focus entirely on a plane normal to the optical axis of the objective optical system but cannot be brought into focus entirely on a plane other than the normal plane, e.g., a side surface of an object having a wide length which extends at an angle relative to a plane normal to the optical axis of the viewing optical instrument. In such conventional viewing optical instruments, particularly within a short distance range, the eyepiece can be brought into focus only on a small part of a surface if the surface extends at a large angle relative to a plane normal to the optical axis of the objective optical system of the viewing optical instrument, so that the focal point needs to be re-adjusted many times if the user desires to observe the entire large-angled surface. For instance, when the user looks at a wide blackboard with a conventional viewing optical instrument from the right or left front corner in a large auditorium, or when the user monitors a wall in a large square room from any of the four corners of the room, the in-focus area in the field of view will be narrow, which is quite inconvenient.